For over a century now, manufacturers have used assembly lines to provide reliable and consistent work flow of workpieces and material through various manufacturing operations to create an end product. These assembly lines widely vary depending on the desired end product as well as the type of manufacturing process; however, almost all have some common features. Most assembly lines include a conveyor system such as a chain conveyor system, power and free conveyor system or any other type of material conveyor system that is designed and installed permanently into the facility. To provide consistent work flow, most conveyor systems are configured to couple to or support a workpiece at a substantially uniform predetermined distance and configured to move along a path at a set speed. Each conveyor system is configured to keep the workpiece consistently spaced no matter the speed, acceleration, deceleration, stop, or start conditions. Typically, a single speed control will control the speed of the interconnected workpieces along the conveyor system, such that it is simple to match the speed of all workpieces to a desired takt time. As all workpieces are securely coupled in some form together, consistent travel of all workpieces automatically occurs. Most conveyor systems also require a chain, belt, or track that forms the path, couples all objects together and is installed permanently into the manufacturing facility. As such, traditional assembly lines and conveyor systems work extremely well at providing consistent through put of work in manufacturing operations, especially where the timing of workpieces entering and exiting a particular work station is important, but they have been generally expensive to initially install and also lack flexibility for easy reconfiguration. The presence of the conveyor equipment often prevents access to the part from all sides and prevents workers from crossing the conveyor path.
Automatic guided vehicles or AGVs are commonly used in many industries to provide material handling and transport various loads without a human operator. The term “AGV” is commonly used to refer to robust vehicle designs having any number of available automated guidance systems. The term “AGC” is also commonly used to refer to less robust vehicles such as automatic guided carts which are similar in nature to AGVs, however, are typically designed to carry smaller loads. Throughout this application, including the claims, the term “AGV” or automatic guided vehicle shall mean and include both AGVs and AGCs as well as any other vehicle that is capable of being autonomously guided. Autonomous guidance and AGVs do not include vehicles being remotely controlled by human operators, but instead must be capable of following a path or route without human intervention.
Current AGV designs generally include a frame with at least two wheels, one of which may be a drive wheel. The drive wheel provides motion to the cart and may also be a steerable drive wheel but in some instances, the non-driven wheels may instead or in combination, act as the steerable wheel. An AGV requires a guidance system to control its movement. A variety of guidance systems are available for use in AGVs including wire guidance, laser guidance, magnetic tape guidance, odometer guidance, inertial guidance, dead-reckoning, optical guidance and a variety of other less used guidance systems. Each type of guidance system generally has associated positives and negatives. For example, an inertial guidance system may be susceptible to tracking errors where the travel distance and direction measured by the AGV differs from the actual distance and direction of travel due to wheel slip on the supporting surface. A variety of methods have been proposed to minimize such tracking errors but the tracking errors may compound over long travel distances. As such, many AGVs include backup or secondary guidance systems which may provide a position or status check, and as such be used to correct for any errors. For example, way point reference markers may be added to the system such as magnetic paint, radio frequency identifier tabs and optical tags to allow an AGV to update its position to a correct position and thereby minimize any guidance errors. There are systems today that use sensors that detect features already in the environment. These systems do not require the addition of reference markers.
Due to the variety of potential errors introduced by at least one of the guidance and drive systems, AGVs have primarily been used in facilities only for the moving of materials such as delivery of raw materials to an assembly line, the removal of finished materials to storage, and from storage to distribution and shipping. In these instances, the AGV may be programmed with a specific path that an individual AGV travels along, but none of the issues associated with a conveyor system in a manufacturing operation are of concern. In addition, while AGVs may be part of material handling system and work in cooperation with the overall system, they do not individually coordinate movement in the facilities other than avoiding potential collisions between AGVs. As such, AGVs have generally not worked in coordination but instead each perform their own unique task and only coordinate to prevent collisions, or move material along desired paths such that parts A are coordinated to arrive with a parts B at a particular work station.
Some manufacturers have tried to use automatic guided vehicles in manufacturing operations or in various facilities as a replacement for typical conveyor systems although until the present invention, no manufacturer has successfully implemented such a system. Coordinated movement of AGVs in a cost-effective and reliable manner, similar to conveyor systems was not yet possible. For example, if an AGV system was to coordinate all AGVs using a central broadcast time signal, the time signal to each AGV has complications with respect to starting and stopping, which is frequently required in a manufacturing facility. More specifically, there are many timing problems associated with identifying the exact time a vehicle stops or starts due to inherent latencies in communication systems. Without an exact time the vehicle stops or starts, it is unknown where a particular vehicle is in relation to other vehicles and in relation to the external manufacturing operation. As such, problems may occur in restarting the system, such with spacing between the AGVs. As such, a group of AGVs was not capable of being configured to match a desired takt time.